Piston machines are provided for many purposes and in a wide variety of constructions and operating modes and such machines can be utilized as internal-combustion or expansion engines, as compressors or pumps, or wherever expansion of a fluid is intended to drive a piston or the reciprocation of a piston is intended to compress or displace the fluid.
It is common practice to provide the machine with one or more pistons which can be reciprocatable in respective cylinders, this term being used to designate a chamber whose volume can vary with reciprocation of the piston.
Where the piston is linearly reciprocated, it is usually connected via a piston rod directly or indirectly to a crankshaft so that, depending upon the nature and construction of the machine, the crankshaft can be driven by the reciprocation of the piston or the reciprocation of the piston can drive the crankshaft.
In an internal-combustion engine, for example, a fuel/air mixture is introduced into the piston chamber or cylinder and is fired during an Otto or Diesel cycle, with the expanding gas mixture driving the piston to apply torque to the crankshaft.
In a compressor, however, rotation of the crankshaft displaces the piston to compress a fluid. In both cases intake and/or exhaust valves may be provided in a cylinder head or the engine or compressor body to control the flow of the fluid into and out of the compartment.
Because of the arrangement of the piston, piston rod and crankshaft were hitherto considered to be necessary to perform useful work with such machines, the overall size and especially the height of the machine was considerable. The height of the machine can be reduced by limiting the stroke of the piston but this, in turn, provides a limitation on the displacement of the machine and hence its capacity. Furthermore, for tolerable efficiencies, the strokes cannot be decreased beyond a certain limit and thus relatively large engine or compressor heights have had to be tolerated heretofore.
The piston rod construction also has the disadvantage that relatively large amounts of energy and force must be transmitted by the piston rod and its bearings and frequently the piston rod was subject to breakage or considerable wear which resulted in high maintenance cost for the machine, interruption in the operations thereof and the like. The danger of breakage and other difficulties of a similar nature increases with the length of the piston rod. Attempts to eliminate these dangers by making the piston rod especially massive have created their own problems with respect to wear and cost.
It is for this reason that investigations have been undertaken into so-called rotary-piston engines; such engines generally eliminated the piston rods and afforded a more direct approach to the transmission of torque from the piston to the shaft and vice versa of the machine.
Increasing experience with rotary-piston engines and compressors has shown that, while such machines are more compact than the piston-rod machines described previously and are capable of transmitting torques with reduced inertial effects and without some of the disadvantages of piston rod machines, the rotary-piston machines have problems regarding sealing of the compartments which cannot readily be overcome.